Yersinia pestis, in three pandemics, resulted in some 200 million plague deaths and is still endemic throughout the world resulting in sporadic infections. Due to its inability to lead a saprophytic life and its residence in many rodent populations, plague is one of the most feared of zoonotic diseases caused by an obligate animal-human pathogen. The plague bacillus began to be used as a biological weapon at least 800 years ago and is today one of the more likely biological threats. Because of these considerations, we propose to: (i) Construct and evaluate recombinant attenuated Salmonella Typhimurium vaccines (RASV) synthesizing Y. pestis KIM antigens in vivo after oral immunization of mice to identify antigens or combinations of antigens that stimulate protective immunity against all three human pathogenic Yersinia species, including virulent Y. pestis CO92. (ii) Construct and evaluate a recombinant attenuated S. Paratyphi A to deliver multiple protective Y. pestis KIM antigens. If it is found that different antigen delivery modes are required to induce protective immunity to Y. pestis challenge, we will construct all deemed necessary recombinant vaccines to be administered as a cocktail. (iii) Conduct experiments to establish all safety, efficacy and immunogenicity features and provide data to secure an IND license from the FDA, make Master Seeds and validate their stability. We will also develop our Master File, prepare and fully characterize candidate vaccine Master Seeds for stability and safety, prepare and submit protocols for IRB approvals, submit information necessary to obtain INDs, and perform any other work needed to arrange that the best candidate vaccines be clinically evaluated in human volunteers. We will provide research support during the proposed clinical trials.